1. Field of the Invention
This invention relates to an output level control circuit for controlling the output level of an amplifier or a like circuit, and more particularly to an output level control circuit which varies the output level of a transmitter of an automobile telephone system or a like system over a wide dynamic range.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example of a conventional output level control circuit of the type mentioned. The output level control circuit shown includes amplifier 21, coupler 22, detection circuit 23, level control circuit 24, comparison circuit 25, amplifier control circuit 29 and control signal generation circuit 30. Amplifier 21 amplifies a signal SS inputted thereto from input terminal 31 and outputs amplification output SQ through output terminal 32. Coupler 22 outputs coupler output SR which increases in proportion to the amplification output level in accordance with a fixed coupling amount relative to amplification output SQ. Coupler output SR of coupler 22 is detected by detection circuit 23, and detection output SD of detection circuit 23 is weighted by level control circuit 24 and outputted as weighted detection output WSD.
Control signal generation circuit 30 receives setting signal S10 for the level of output SQ from a control section (not shown) through control terminal 33 and generates and supplies reference level S11 in accordance with setting signal S10 to comparison circuit 25. Comparison circuit 25 compares output WSD of level control circuit 24 and reference level S11 to generate error signal G11 and supplies error signal G11 to amplifier control circuit 29. Amplifier control circuit 29 generates control voltage G12 in accordance with error signal G11 in order to adjust the output level of amplifier 21 to the level indicated by setting signal S10. A closed loop is formed to control the output level of amplification output SQ in this manner.
In an automobile telephone system or a like system, the transmission output to be sent from a base station must be continuously varied to an optimum level as an automobile telephone moves. The level of the transmission output must be varied over a wide range. In this instance, the conventional output level control circuit is disadvantageous in that it is not easy to obtain the output of the amplifier with a high degree of accuracy at the upper limit or the lower limit of the level of output SQ because of the non-linearity of the characteristics of the detection circuit.
To describe this disadvantage in more detail, detection output SD of detection circuit 23 does not increase in proportion to coupler output SR from coupler 22, and the characteristic between outputs SD and SR is consequently indicated by a non-linear characteristic curve as shown in FIG. 2. Particularly within the range (between RL and RM in FIG. 2) of the level of output SR below boundary level RM, that is, on the low-level side, the characteristic curve becomes nearly horizontal. Since the characteristic curve of FIG. 2 can generally be represented as two linear lines shown in FIG. 3, the following description proceeds with reference to FIG. 3 for a simplified description.
In order to cause detection output SD (the line SD is composed of lines SD.sub.1 and SD.sub.2 in FIG. 3) to increase in proportion to output SR as far as possible within the overall range (RL to RH) of output SR in the characteristic indicated in FIG. 3, level control circuit 24 must, for example, raise the amplification factor (gain) on the low-level side of output SR. However, level control circuit 24 of the conventional output level control circuit of FIG. 1 is defective in this regard since it applies the same weighting to output SR over the entire range of the output level. For example, if the inclination of the linear line (SD.sub.1) on the low-level side of output SR is increased with the intention of bringing the low-level side of output SR close to its ideal condition by increasing the amplification factor, the inclination of the high-level side (RM to RH) of output SR above the boundary level also increases, thereby distancing the output SD.sub.2 on the high-level side of output SR from its ideal condition. In contrast, if the inclination of the linear line on the high-level side of output SR is decreased with the intention of bringing the high-level side close to its ideal condition, the inclination of the linear line on the low-level side decreases, thereby distancing the output SD.sub.1 on the low-level side of output SR from its ideal condition.
Accordingly, it is difficult to allow output SD to approach its ideal condition within the overall range of output SR, and the control of output SD corresponding to output SR remains a compromised weighting control. Meanwhile, according to another conventional method, it seems possible to provide level control circuit 34 with an input-output table in the form of ROM or a like element so as to ensure linearity. However, this method requires an expensive device such as ROM and is therefore not desirable for reasons of cost.